Field of the Invention
The present invention relates to a humidification control method for a fuel cell and a fuel cell system. The fuel cell is formed by stacking an electrolyte electrode assembly and a separator. The electrolyte electrode assembly includes a cathode, an anode, and an electrolyte interposed between the cathode and the anode.
Description of the Related Art
For example, a fuel cell includes a membrane electrode assembly (MEA) and separators sandwiching the MEA. The membrane electrode assembly includes a cathode, an anode, and an electrolyte membrane (or an electrolyte) interposed between the cathode and the anode. The electrolyte membrane is a polymer ion exchange membrane. In use, a plurality of the fuel cells are stacked together to form a fuel cell stack, e.g., mounted in a vehicle.
In this type of the fuel cell, in order to stabilize the power generation performance, it is required to maintain the electrolyte membrane to be in a suitable humidified state. For example, if the membrane electrode assembly is dried, the internal resistance of the fuel cell becomes large, and the output voltage is decreased. If excessive water is present in the membrane electrode assembly or reactant gas flow fields, a phenomenon (flooding) which inhibits, e.g., diffusion of reactant gases occurs, and the output voltage of the fuel is decreased. In an attempt to address the problem, in a fuel cell system which controls the fuel cell, at the time of power generation, humidification control is implemented by measuring the quantity of water (water content) in the fuel cell system.
In this regard, it is known that the water content of the fuel cells changes in accordance with impedance of the fuel cells. Therefore, in the fuel cell system, alternating current is applied to the fuel cells at different frequencies, and impedance is measured at a plurality of frequency points. Based on the results of this impedance measurement, the water content of the fuel cells is estimated.
For example, fuel cell systems disclosed in Japanese Laid-Open Patent Publication No. 2003-086220 and Japanese Laid-Open Patent Publication No. 2009-004299 adopt structure where alternating current is applied at two different frequencies to calculate alternating current impedance where the frequency varies from a high frequency (ω=∞) to a low frequency (ω=0).